1. Field of the Invention
This invention relates to zoom lens systems, and; more particularly; to compact zoom lens systems suited to still cameras, cine cameras and video cameras.
2. Description of the Prior Art
It has already been known in the art to provide a zoom lens system comprising four lens components of which the front three are moved axially to effect zooming in Japanese Laid-Open patent application No. SHO 54-30855. These components are a first component, counting from the front, of positive power, a second component of negative power, a third component of positive power, and a fourth component of positive or negative power. As zooming is performed from the wide angle end to the telephoto end, the first and third components move forward, while the second component moves rearward.
In general, zoom lens designer works out a diameter of the first lens component by taking into account, in most cases, a rough list of the following factors:
(a) The axial beam diameter for the telephoto end at full open aperture. PA1 (b) The ability to admit as large an oblique beam as the axial beam for the wide angle end when the object is at a minimum distance. PA1 (c) The ability to admit as large an oblique beam as the axial beam for the telephoto end when the object is at the minimum distance. PA1 (d) The ability to admit as large an oblique beam as the axial beam for the intermediate position when the object is at the minimum distance.
Of these, factor (a) takes on a certain value when the zoom lens is specified, and, therefore, unavoidably affects it. But this usually gives rise to no serious problem. Since factors (b) and (c) depend on the paraxial power distribution at the start point, a change of the power distribution is required.
With the use of the type described in connection with the aforesaid prior art example, where the first lens component moves forward, factor (d) becomes important for determining the value of the diameter of the first lens component in most zoom lenses. As zooming goes from the wide angle end, the distance between the first and second lens components increases rapidly particularly in the vicinity thereof with decrease of the image angle. Therefore, this type of zoom lens is associated with a drawback that the diameter of the front or first component becomes larger and the minimum object distance becomes longer. To reduce the lens diameter, the method of strengthening the refractive power of each of the lens components is very often employed. But this produces an alternative problem that the optical performance deteriorates badly, and the tolerances to which the manufacturing operations are held become more severe. Another method of moving the stop with zooming may be employed in order to insure that the oblique beam for the intermediate focal length position is admitted when the object is at the minimum distance. But this has a drawback that the operating mechanism is strained.
FIG. 1(a) illustrates paraxial power distributions of a prior art example of the zoom lens system along with the loci of movement of all zoom components thereof. FIGS. 1(b) and 1(c) are graphs representing a variation of the separation e1 between the first and second components 1 and 2 with the amount of movement M3 of the third component 3 as measured from the wide angle end, and depicting a partial differential equation of first order therefor; respectively. Symbols e1W and e1T denote the separation between the first and second components when in the wide angle end and telephoto end, respectively.
The rate of variation of the separation e1 between the first and second components 1 and 2 against the amount of movement M3 of the third component 3 is steeper in the wide angle end than in the telephoto end. In other words, a partial differentiation of second order .delta..sup.2 e1/.delta.M3.sup.2 &lt;0. In such case, the diameter of the first lens component must be increased. Otherwise, it would be difficult to insure that an oblique beam as large as the axial beam is admitted for the minimum object distance in the region from the intermediate focal length position to the wide angle end, because the separation between the first and second components 1 and 2 increases at a far higher a rate than that of decrease of the image angle as zooming goes from the wide angle end. As a result, the zoom lens system tends to increase in the bulk and size thereof.